PRISMS. 



191 



be shown by the engraving, where the rays forming the 

 centre portion of the diverging beam are brought to paral- 

 lelism by the lens, and those that ascend and descend much 

 are partly refracted, but mainly reflected, by the prisms 

 placed in proper positions for this purpose, as shown in 

 section in the engraving. 



EFFECT OF TIIE I'RIGMS. 



You will observe from the engraving that the light is 

 reflected from the under side of the upper surface of the 

 prism, where we should naturally think it would emerge. 

 It is very remarkable that it should be thus reflected back 

 through the interior of the glass again, instead of going 

 out into the air ; but such is the fact. We can see a strik- 

 ing example of reflection of this kind by means of a tum- 

 bler of water. Fill a tumbler nearly full, then hold it up 

 carefully above your head, and look up at the under surface 

 of the water ; you will find that you can not see through 

 it to what is above. If, while you hold the tumbler in one 

 hand, you hold the finger of the other hand above the top 



